Among the high-temperature alloys based on refractory metals for aero-space and nuclear applications, etc., various tungsten- and molybdenum-alloys containing high amounts of rhenium have been considered and used for a long time. Thus, it is known that the properties of such alloys are greatly improved by the so-called "rhenium effect", which means i.a., that a rhenium addition simultaneously improves strength, plasticity and weldability; lowers the ductile-to-brittle transition temperature of wrought products; and reduces the degree of recrystallization embrittlement.
The greatest improvement in properties are obtained with additions of 11 to 50 wt % Re in the case of Mo. Particularly useful alloys have been found in the range of 40-50 wt % Re and two commercial alloys have the compositions Mo-41 wt % Re and Mo-47.5 wt % Re.
With ever increasing demands and requirements upon the engineering and structural materials, it has been shown, however, that the alloy with 41% Re has a ductile to brittle transition temperature of about -150.degree. C. (about 125K) which is too high for most space applications. Furthermore, the alloy with 47.5% Re corresponds to a supersaturated solution of Re in Mo and when exposed to temperatures between about 1075.degree.-1275.degree. C. (about 1350-1550K) an embrittling sigma (.sigma.) phase (Mo Re) will be precipitated-decreasing the otherwise excellent low temperature ductility to the same order of magnitude as for the Mo-41 wt % Re.
Consequently, neither of the two above described Mo-Re alloys nor any other known Mo-Re alloy fulfills the requirements in the aero-space applications regarding said kind of material being necessary today.
Old technical information on Re-Mo alloys exists in the literature, but there are several incorrect data included, which makes it difficult to interpret the information in an accurate way. Thus, there are phase diagrams indicating that the sigma phase does not exist at temperatures below about 1150.degree. C. (about 1425K). The fact, is, however, that the sigma phase is stable down to OK (-273.degree. C.) but does not form in reasonable time periods at temperatures below about 1125.degree. C. (about 1400K) because of slow diffusion rates. Furthermore, there are old data regarding the effect of rhenium alloying on the ductile-to-brittle bend transition temperature of molybdenum showing that, e.g., Mo-50 Re has a constant ductile behavior, while Mo-45 Re has an average ductile-to-brittle transformation temperature of about -180.degree. C. (about 95K). Said data do not take into consideration, however, that Mo-alloys with more than about 45% Re may get embrittled in welding and other joining processes, used in fabricating components.